Using the technique of isolated rabbit renal tubule perfused in vitro studies are to be performed to investigate calcium transport in proximal convoluted and straight tubules, thin descending and thick ascending limbs of Henle, distal convoluted tubule, and in cortical and medullary collecting tubules. The influx and outflux coefficients for calcium will be measured with 45Ca and evidence for passive or active net flux sought from simultaneous measurements of potential. The effects of hormones, other ions and various metabolic and dietary manipulations will be examined.